I'm pretty sure baking the drivers in to the boot/install.wim file is the only way to go. Did you make sure to grab the drivers specific to your board and not the ones on that instruction page?
Yes, I used the ones from AMD's site for my mother board.
Not being funny but you really need to look at moving to 10, I know you might not prefer it but by all means use hokey third party tools to make it look like 7 and strip out the inbuilt functionality but 7 is just not well supported on modern hardware and its out of support in 18 months time.
Yeah I know, one day I have to but if I can avoid it for a while longer I prefer that.
I actually managed to install win 7 finally, so I WILL avoid Win 10 for a while longer !!

Had to be a bit cheeky though and use another manufactures tool to make it work. The Windows USB Installation Tool from Gigabyte themselves did not manage to burn the USB drivers into the Win7 USB Installation ISO. But the
Asrock Windows 7 USB Patcher sorted it out just fine (once at their site, have to use drop down menu to pick Win7 64 Bit, and then the patcher is at bottom). I did switch the drivers to the ones from Gigabyte though, not sure if it was needed but thought I do that, to be on the safe side.
Once the installation was sorted, I could not use the mouse though (USB input mouse, and USB drivers was not working once win7 was running after installation, not quiet sure why though.

). To solve that I had to use my old internal sata DVD player as can be seen in the picture.
No room for it due to having a 360 Radiator at the front so had to connect it that way, it worked though !
By doing that I could access the Gigabyte sotware disc for the mother board, then opening it and tabbing my way around until I could find the AMD Chipset Drivers, pathway was folder Chipset then Bristol folder. There after run the setup and it installed all the needed drivers and now the USB ports working and so does the mouse.

Tomorrow will be less fun by installing more drives and get all programs up and running that I had on my old PC.

Edit: Ohh and I forgot, have to do all the windows updates too.
But for now I am well chuffed though !!
Edit 2: Thought I run the Windows Updater to see what needed to b done. I got a small shock though !